FIG. 1 is a diagram illustrating an exemplary configuration of a typical wavelength division multiplexing (WDM) optical transmission system. In this WDM optical transmission system, a WDM signal light, composed of multiplexed optical signals each having a different wavelength, is bidirectionally transmitted between opposite WDM optical transmission devices 1 through optical transmission lines (WDM lines) 2. Each WDM optical transmission device 1 is further connected to two or more client devices 3 via optical transmission lines (client lines) 4. The WDM optical transmission device 1 includes, for example, as illustrated in FIG. 2, an optical interface device 10 and an optical wavelength multiplexing-demultiplexing device 30, and performs inter-conversion between optical signals respectively carried on channels CH1 to CHn through the client lines 4 and a WDM signal light that is transmitted through the WDM line 2.
The optical interface device 10 has a plurality of optical units 10A, 10B, etc. For example, the optical interface 10A converts optical signals carried on four channels, which are received from the client lines 4, to an optical signal suitable for transmission on the WDM line 2, and performs reverse conversion thereof. Furthermore, the optical interface 10B converts an optical signal carried on one channel, which is received from the client line 4, to an optical signal suitable for transmission on the WDM line 2, and performs reverse conversion thereof.
Specifically, in optical signal conversion by the optical interface 10A, optical signals, which are carried on four channels CH1 to CH4 through the client lines 4, are respectively received by optical transceivers 11 which are compliant with MSA (Multi Source Agreement) specification, and each optical transceiver 11 generates appropriate client signals (a data signal and a clock signal) for each optical signal. Each client signal generated by each optical transceiver 11 is converted so as to have a frame format for an OTU (optical channel transport unit) by a digital wrapper 13 that is defined by the ITU-T G.709 (Interfaces for the optical transport network, OTN). Then, a data string, that was contained into an OTU frame by the digital wrapper 13, is supplied to an MSA-compliant optical transceiver 15, and the MSA-compliant optical transceiver 15 modulates a light within the WDM wavelength band according to the data string and outputs the resultant optical signal to the optical wavelength multiplexing-demultiplexing device 30. In the optical wavelength multiplexing-demultiplexing device 30, optical signals of different wavelengths from the optical interfaces 10A, 10B, and so on, are multiplexed by a multiplexer 31, and the resultant WDM signal light is amplified to a desired level and is supplied to the WDM line 2 by a post-amplifier 33.
In the process performed by the digital wrapper 13 above, if a clock signal corresponding to an optical signal received from the client line 4 is asynchronous with a clock signal corresponding to an optical signal to be transmitted to a WDM line 2 side, the client signal is framed by synchronizing this client signal with a standard clock signal that is generated by a fixed oscillator, etc., within the optical interface. At this time, a stuffing process is performed to compensate for a difference between a bit rate (clock frequency) of the client signal and a standard clock frequency. In the stuffing process, an NJO (negative justification opportunity) byte and a PJO (positive justification opportunity) byte, both defined for an overhead of an OPU (optical channel payload unit) within the OTU frame, are used. According to the difference between a clock frequency of a client signal and a standard clock reference, either positive stuffing for inserting a justification byte (zero) or negative stuffing for accommodating extra data of a client signal is performed. Furthermore, when a stuffed signal is processed by the digital wrapper 13 on receiving side, a client signal that is similar to that on the transmitting side is recovered by performing a destuffing process based on the aforementioned OPU overhead information.
In the WDM optical transmission device 1 as described above, a client-side optical signal supplied to the optical interface device 10 has a clock frequency which includes a deviation within ±20 ppm according to ITU-T Recommendations. Furthermore, an optical signal available for Ethernet (registered trademark) may also be input to the optical interface device 10. For example, an optical signal available for 10 Gbit/s Ethernet (hereinafter abbreviated as “10 GbE”) has a clock frequency which includes a deviation within ±100 ppm according to IEEE 802.3ae.
A prior art technique for accommodating such a frequency deviation of a client-side optical signal is, for example, disclosed in International Publication Pamphlet No. WO 2007/072921 which proposes, regarding a stuffing process, to enable synchronization with a client-side optical signal within a large frequency range by adding a new overhead to each client signal, and using two or more defined stuffing bytes in cooperation with the new overheads.
However, even if such a conventional stuffing technique described above is applied to an optical interface device, when a deviation of a clock frequency of a client optical signal that is actually supplied to the optical interface device is outside of the defined range above due to occurrence of trouble in a system, a client device which receives the optical signal at a terminal of the line may detects an error. In this case, a conventional WDM optical transmission system suffers from the problem that locating a position in which an abnormal frequency condition occurs is difficult.
For example, in the forgoing exemplary configuration illustrated in FIG. 1, suppose that a deviation not within ±100 ppm occurs in a clock frequency of a 10-GbE optical signal to be transmitted to the WDM optical transmission device 1 from an optical transmitter (TX) of the client device 3 through the client line 4. In this case, when each optical module provided in each of the WDM optical transmission devices 1 which are respectively disposed at the opposite ends of the WDM line 2 has a proof strength with respect to the aforementioned deviation of the clock frequency whereas an optical receiver (RX) in the client device 3 disposed at a terminal end of the client line 4 on receiving side has no proof strength with respect thereto, the receiving client device 3 detects an error, an error detected by a client device 3 on receiving side. At this time, however, since each WDM optical transmission device 1 successfully completes its process, the matter that some error occurs in the clock frequency of a client-side optical signal to be supplied to the transmitting WDM optical transmission device 1 cannot be determined. Furthermore, since about ±160 ppm margin is provided for a stuff amount that is processed by an optical interface within each WDM optical transmission device 1 due to application of the conventional technique described above, no error resulting from a stuffing process limit occurs.
In addition, in the conventional WDM optical transmission device 1, the optical interface 10B (FIG. 2) for converting a 1-channel optical signal received from the client line 4 may use a clock signal, which was used for conversion of a client-side optical signal, as a reference clock signal when recovering a data signal and a clock signal by use of a received light (an optical signal received from the WDM line 2) on the opposite line side within the same optical interface. In this case, when a deviation of a clock frequency of a client-side optical signal increases, a frequency deviation of a reference clock signal in an optical reception process on the aforementioned opposite line exceeds the proof strength. This poses a problem because it may cause error occurrence on the opposite line, as well as the above-described error detection problem at the line terminal end.